High concentrations of nitric oxide (NO) regulation of NO synthase (NOS) activity is essential for minimizing the effects of cytotoxic and genotoxic nitrogen oxide species. We have shown previously that NO-induced p53 protein accumulation, down-regulates basal and cytokine-modulated inducible NOS (NOS2) expression in human cells in vitro, and that p53-null mice have elevated NOS2 enzymatic activity. Our investigation of primary colon tumors establishes a strong positive relationship between the presence of NOS2 in tumors and the frequency of G:C to A:T transitions at CpG dinucleotides. These mutations also are common in lymphoid, esophageal, head and neck, stomach, brain, and breast cancers. Increased NOS2 expression has been demonstrated in four of these cancers. Tumor-associated NO production may modify DNA directly, or may inhibit DNA repair activities, such as the recently described human thymine-DNA glycosylase, which has been shown to repair G:T mismatches at CpG dinucleotides. Because NO production also induces the accumulation of wild-type p53, the resulting growth inhibition can provide an additional strong selection pressure for nonfunctional, mutant p53. NO may, therefore, act as both an endogenous initiator and promoter in human colon carcinogenesis, and specific inhibitors of NOS2, as demonstrated recently in an animal tumor model, may have important chemopreventive potential in human colorectal cancer. Because the frequency of p53-mutated alleles in nontumorous human tissue may be a biomarker of oxyradical damage and identify individuals at an increased risk, we have determined the frequency of p53-mutated alleles in ulcerative colitis (UC), an inflammatory bowel disease. The p53 mutation load was increased, indicating the genotoxic effects of chronic oxidative stress in this cancer-prone disease.